The present invention relates to a convergence correcting device for an in-line type color picture tube. More particularly, this invention relates to a convergence correcting device for use in a self convergence type color picture tube, in which three electron beams emitted from three electron guns arranged in an in-line form, are automatically converged by vertical and horizontal deflecting magnetic fields.
It is well known, that in front of the neck portion of a picture tube, is a deflecting means which deflects the electron beams of the electron guns to the screen.
This deflecting means is situated such that a vertical deflecting coil is installed on the outside of a funnel shaped separator in order to deflect the electron beams in the vertical direction, and a horizontal deflecting coil is installed in the inside of the funnel shaped separator in order to deflect the electron beams in the horizontal direction. A specified voltage level is applied to the vertical and horizontal deflecting coils to establish magnetic fields required to deflect the electron beams.
In a color picture tube, three electron beams pass the holes formed in a shadow mask before they land on the respective corresponding phosphorescent dots. The combinations of these landed beams form a picture in color registration. However, if such conditions are not met, then the color of the picture becomes degraded.
Such deviations can occur over the whole picture, or they can occur partially along the periphery of the picture.
If they occur over the entire picture, the advancing directions of the three electron beams are not correct. To correct the picture, the advancing directions of the three beams must be corrected. This is done by ensuring that the three electron beams which have passed through the respective holes of the shadow mask land on the correct positions on the 3-color dots.
To allow for correction of the three beams' landing positions, the vertical and horizontal deflecting coils are equipped with a magnet capable of varying the direction and the intensity of the deflecting magnetic fields, and electric currents having special wave patterns must be supplied to these vertical and horizontal deflecting coils.
An example of such a device uses a conventional in-line type self convergence deflecting yoke situated so that the deflecting magnetic field of the horizontal deflecting coil is shaped into a pin cushion form, and the deflecting magnetic field of the vertical deflecting coil is shaped in a barrel form, in order to correct the directions of the electron beams.
If the beam deflecting angle of a color picture tube is expanded to an angle greater than 90 degrees in order to make the electron beams converge to an acceptable degree, a pin cushion deformation or a barrel cushion deformation occurs to the upper and lower rasters, thereby making such a method impractical. On the other hand, if the deformations of the upper and lower rasters are adjusted to the optimum, a forward cross misconvergence or a reverse cross misconvergence occurs, also making this method impractical.
The problems of the convergence characteristics and the raster deformations present technical difficulties that have to be solved.
Japanese Patent Application No. Sho-56-91275 and 56-111650 propose devices which are allegedly capable of overcoming the problems described above. However, in these devices, the operating range of the controlling magnetic fields is limited which results in the DC bias level being too small and requires the structure to become too complicated. Laid-Open Japanese Patent Application No. Sho-60-125069 discloses another device which is allegedly capable of overcoming the problems described above. In this device, however, the difference between the misconvergence amount of the middle portion of the picture showing negligible vertical deflections and the misconvergence amount of the upper and lower portions of the picture showing maximum vertical deflections is approximately 0.5 mm. Therefore, it is not suitable for picture tubes in which the pitches of the phosphorescent dots are ultra-fine to below 0.3 mm.
The devices described above also have fixedly installed magnets making it impossible to adjust the DC bias in those devices.